Field of the Invention
The present invention relates to secondary batteries such as a lithium ion secondary battery.
Description of the Related Art
A secondary battery takes out electrical energy by converting chemical energy to the electrical energy through a redox reaction at a cathode and an anode, or accumulate the electrical energy through a reverse reaction thereof. The secondary battery has been used as a power source in various devices.
Recently, due to rapid market growth of a device such as a laptop computer or a smartphone, demand has been increasing for dramatically improving energy density and output density of the secondary battery used for the device. In order to alleviate the power shortage after Great East Japan Earthquake, expectations have been rising for development of a large scale and large capacity secondary battery. For the purpose of meeting the above demands, there has been actively developed a lithium ion secondary battery that includes an alkaline metal ion (e.g., a lithium ion) as a charge carrier and utilizes an electrochemical reaction resulting from donating and accepting electrons by the charge carrier.
Most of the lithium ion secondary batteries include an electrode material on a cathode side (cathode active material) having smaller discharge capacity (Ah/Kg) than an electrode material on an anode side (anode active material). This is a main reason why the lithium ion secondary batteries cannot be increased in capacity. Lithium ion secondary batteries currently available in the market include a metal oxide having a large specific gravity as the cathode active material, which is problematic in terms of insufficient battery capacity per unit mass. Therefore, many attempts have been made and reported to develop a large capacity secondary battery using a lighter electrode material, that is, an organic compound (e.g., see U.S. Pat. No. 4,833,048, Japanese Patent No. 2715778, Japanese Examined Patent Publication No. 7-85420, Japanese Patent No. 4687848, Japanese Unexamined Patent Application Publication No. 2010-80343, “Chemical Physics Letters, 359, (2002) 351-354” and “Nature Materials, 10, (2011) 947-951). However, the art described in the above related art references has resulted in unsatisfactory secondary batteries.
Recently, there has been suggested a charge and discharge mechanism utilizing a redox reaction of quinone by introducing, as a side chain, a functional moiety containing a quinone skeleton (quinone moiety) into a polymer main chain (e.g., see Japanese Unexamined Patent Application Publication No. 2013-20760). In this art, due to anionic stability of the quinone moiety, charge and discharge proceed through a rocking chair mechanism in which only Li ions are transported. Therefore, the charge and discharge can be stably performed without varying a concentration of an electrolytic solution during the charge and discharge.
However, in the art described in Japanese Unexamined Patent Application Publication No. 2013-20760, the discharge capacity per unit mass is decreased because the quinone moiety is introduced into the polymer. Therefore, there is a problem that the quinone cannot exhibit the discharge capacity sufficiently.